Nearly five years have passed since the explosion of the Deepwater Horizon oil rig set off one of the largest oil-spill disasters in U.S. history. The April 2010 blast killed 11 workers and discharged millions of gallons of oil into the Gulf of Mexico. In response to the spill, about 750,000 gal of dispersants were injected underwater into oil gushing from the well. Another 1 million gal were sprayed from airplanes onto oil slicks.